Future mobile and wireless communication networks will support a variety of services. Particular services, such as services enabling traffic safety applications, have extremely strict latency requirements. A response time for a terminal device using such a delay-sensitive service, i.e. the time period from the time point, at which a terminal device initiates a transmission to the base station, until the time point, at which the transmission is received at the base station, and is optionally confirmed by an application server, needs to be a short time period, e.g. in the order of milliseconds. Within this short time period, the network should accomplish the whole uplink transmission procedure.
Due to its unpredictable nature, an uplink transmission procedure is typically a RA procedure, in which resource allocation to transmitters, for example terminal devices, cannot be coordinated in advance. In case that more than one terminal device employs the same uplink resource at the same RA time slot for a transmission to the base station, a collision occurs. This collision usually results in interference such that some or all of the colliding transmissions cannot be decoded correctly. As a consequence, the affected transmission has to be repeated, which leads to additional delay in the procedure.
For the aforementioned delay-sensitive services, the delay caused by a collision can be the most critical issue. The latency of a RA channel in the 3GPP Long Term Evolution (LTE) network often exceeds 100 ms (see e.g. 3GPP, TR 37.868, “Study on RAN Improvement for Machine-Type Communication”, V 11.0.0, September 2011), which is far beyond the actual response time requirements for most delay-sensitive services, e.g., traffic safety services. Additionally, when a Machine Type Communication (MTC) is introduced into the network, the number of terminal devices, which may simultaneously access the network, e.g. the base station, will sharply increase. Consequently, the probability of collisions during the RA procedure increases further, and the above-described latency issue becomes even worse.
Since future wireless networks, e.g., LTE new Releases and Fifth Generation 5G, will also be more versatile, a mixture of delay-sensitive service traffic and delay-tolerant service traffic will increasingly coexist in the radio channels. In view of the above-described collision and delay issues, the RA procedure will thus need to employ some prioritization of the different kinds of service traffic. For instance, in case that a resource request for a delay-sensitive service collides with a resource request for a delay-tolerant service, a higher priority should be granted to the delay-sensitive resource request, so that a retransmission of the delay-sensitive resource request is avoided.
In the conventional Carrier Sense Multiple Access (CSMA) scheme, a transmitting terminal device senses and detects signals from other terminal devices before it starts its actual transmission. The time period, in which the terminal device senses, whether the channel is free, is called a contention window for colliding devices. The priority in this scheme can be implemented by adjusting the contention window size such that specific terminal devices are given earlier chances to transmit than others (see Y. Liu et al., “Design of a scalable hybrid mac protocol for heterogeneous m2m networks” IEEE Internet of Things Journal, Bd. 1, Nr. 1, p. 99-111, 2014 or I. Rhee et al. “Z-MAC: A hybrid MAC for wireless sensor networks” IEEE Transaction for Network, Bd. 16, Nr. 3, p. 511-524, 2008). The CSMA scheme bases on the assumption that one terminal device can detect the signal from other terminal devices in advance. This is a practical limitation, particularly in a cellular network with a cell radius over several hundred meters. That is to say, in order to be able to detect other terminal devices, the terminal devices have to be located close to each other. Otherwise, the CSMA scheme will suffer from the so called “hidden node problem” as known from WLAN.
Therefore, in LTE (see 3GPP, TS 36.331, “Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol specification”, V. 10.1.0, March 2011), preambles are used in the RA procedure. As shown in FIG. 1, in a conventional RA procedure 100, a first terminal device 102 and a second terminal device 103 (denoted as UE1 and UE2, respectively, in FIG. 1) each transmit a preamble as a resource request in advance of their actual transmission to a base station 101 (denoted as BS), in order to request a dedicated uplink resource, specifically a time-frequency resource block, from the base station 101.
In particular, once a terminal device 102, 103 in LTE has an unscheduled transmission request, it will start the RA procedure 100 to communicate a resource request for its initial network access. As shown in FIG. 1, the RA procedure 100 comprises four steps.
In a first step, one or more terminal devices 102, 103 transmit a resource request with a randomly selected RA preamble to the base station 101. For instance, as shown in FIG. 1, the two terminal devices UE1 and UE2 each transmit a preamble defined by a signature PA1. A set of all possible preambles is known at the terminal devices 102, 103 and at the base station 101. Therefore, the preamble can also be used as a training sequence and as a signature. The base station 101 can detect different preambles, and can send responses according to individual preambles. In the case of FIG. 1, the base station 101 detects a resource request with the preamble PA1.
In a second step, the base station 101 transmits a RA response in the downlink shared channel in response to the detected preambles. According to the detected preamble of each resource request, the base station 101 assigns an uplink resource to the corresponding terminal device(s) 102, 103. In the case of FIG. 1 the base station 101 grants uplink (UL) resource for the terminal devices UE1 and UE2 sending the preamble PA1.
In a third step, a terminal device 102, 103 transmits its identity and other messages, e.g., scheduling request, to the base station 101 using the resource assigned to it by the base station 101 in the RA response in the second step. In the case of FIG. 1, both UE1 and UE2 recognize that UL resources are granted according to their resource request, and thus both UE1 and UE2 send a message in the granted UL resource.
In a fourth step, the base station 101 echoes the identity of the terminal device(s) 102, 103 it received in the third step.
In a case that—as shown in FIG. 1—two terminal devices 102, 103 choose the same preamble (denoted with PA1 in FIG. 1) for their resource request, the base station 101 cannot distinguish the resource requests from different terminal devices 102, 103. Hence, the same uplink resource will be assigned to both terminal devices 102, 103. In this case, in the third step both terminal devices 102, 103 use the same resource for their actual transmission, and thus a collision occurs. In this collision case, if a transmission sent in the third step cannot be decoded correctly, the corresponding terminal device 102 or 103 will not receive the confirmation from the base station 101 in the fourth step. Thus, it must reinitialize its preamble transmission after a certain time, the so called back-off time.
It can thus be seen that in the above-described first round of the RA procedure 100 shown in FIG. 1, delay-sensitive resource requests do not have any advantage over delay-tolerant resource requests. Only during the retransmission of the preamble, delay-sensitive resource requests may be granted with a shorter back-off time compared to delay-tolerant requests.
In particular, for the RA procedure 100 shown in FIG. 1, a prioritization was proposed via certain back-off schemes (see 3GPP, TR 37.868, “Study on RAN Improvement for Machine-Type Communication”, V 11.0.0, September 2011, and Tarik Taleb and Andreas Kunz, “Machine type communications in 3GPP networks: potential, challenges, and solutions”, IEEE Communications Magazine, Bd. 50, Nr. 3, p. 178-184, 2012. 2012). In these schemes, high-priority terminal devices are assigned a shorter back-off time than low-priority terminal devices. However, as short as the back-off time may be, at least one retransmission is inevitable in case of a collision.
In order to further reduce the potential latency for emergent and delay-sensitive services, LTE also applies a contention-free scheme for RA channel. In this contention-free scheme, particular preambles are reserved only for delay-sensitive services. That is, certain terminal devices are assigned exclusive preambles, which are not shared with other terminal devices. Because the reserved preambles are exclusively used for the specified delay-sensitive services, the collision probability is reduced or even eliminated. However, the number of preambles in a network is typically limited. Further, the reservation of dedicated preambles even reduces the total number of available preambles used for other contention-based accesses. Thus, on the one hand side, if more preambles are reserved for contention-free access, the efficiency of the preamble usage of the contention-based accesses becomes worse. On the other hand side, there may also not be sufficient preambles for all the contention-free accesses of delay-sensitive services.
In addition to the above-described problems of collision and delay, there may also be the problem that the number of required resources to support the terminal devices, which request resources, exceeds the number of available resources that the base station 101 is able to provide. Also in this case, delay-sensitive resource requests do not have any advantage over delay-tolerant resource requests.